The present invention relates to a method for the polymerization of an ethylenically unsaturated monomer or, more particularly, to a method for preventing polymer scale deposition on the reactor walls or other surfaces in the polymerization of an ethylenically unsaturated monomer in an aqueous polymerization medium exhibiting an effect so versatile as hardly to be affected by various parameters such as kind of the monomer polymerized, type of the polymerization, formulation of the polymerization mixture, for example, in respect of the polymerization initiator, stabilizer and the like, and so on.
As is well known, the polymerization of an ethylenically unsaturated monomer or, in particular, vinyl monomer is carried out by the method of suspension polymerization, emulsion polymerization, solution polymerization, vapor-phase polymerization or bulk polymerization. Whatsoever the type of the polymerization may be, one of the most serious problems in practicing the polymerization in an industrial scale is the deposition of the polymer scale on the surfaces of the inner walls of the polymerization reactor and other parts coming into contact with the monomer during polymerization such as the stirrer and the like.
To explain it, polymer scale is deposited almost always on the inner walls of the polymerization reactor, surface of the stirrer and other surfaces coming into contact with the monomer when a vinyl monomer is polymerized in either one of the above mentioned polymerization methods so that several disadvantages are unavoidable. For example, the yield of the polymer product naturally is decreased by the amount of the polymer scale, the cooling capacity of the polymerization reactor is greatly reduced due to the poor heat conductivity of the polymer scale built up on the reactor walls resulting in decreased productivity of the reactor and the polymer scale coming off the reactor walls enters the polymer product leading to the lowered quality of the product. In addition, the polymer scale once built up on the reactor walls and the like can be removed only with great consumption of time and labor which also increases the overall production cost of the polymer product. Moreover, polymer scale is relatively highly absorptive of the unreacted monomer so that the workers for the removal of the polymer scale are exposed to a great danger against their health as is the most serious recent problem in the polymer industry, especially, when the monomer is toxic as is vinyl chloride.
Needless to say, there have been made various attempts to solve this difficult problem of polymer scale deposition and the suspension polymerization of vinyl chloride in an aqueous medium is almost always carried out by undertaking a means for polymer scale prevention. The prevention of polymer scale deposition is performed either by providing coating on the reactor walls prior to the polymerization with certain compounds, for example, such as amine compounds, quinone compounds, aldehyde compounds and the like as an organic polar compound or by adding one or more of these compounds into the aqueous polymerization medium in the course of the polymerization. These methods have their respective disadvantages and there is known no satisfactory method capable of exhibiting sufficient effect of preventing polymer scale deposition. For example, the effect of the method of coating is not so durable that the effect of polymer scale prevention is lost after 5 to 6 runs of repeated polymerization after coating.
Further, the method of coating with a polar organic compound is effective only in the suspension polymerization of vinyl chloride by use of a monomer-soluble polymerization initiator but little effective in the emulsion polymerization by use of a water-soluble polymerization initiator or in the polymerization in which the polymerization medium contains a surface active agent. For example, Japanese Patent Kokai No. 53-13689 teaches a method for preventing polymer scale deposition in the emulsion polymerization of a vinyl monomer by use of a water-soluble polymerization initiator and a surface active agent, according to which the polymerization is carried out in a polymerization reactor having walls coated with an oxidative condensation product of an aromatic amine compound. This method is indeed effective to some extent for preventing polymer scale deposition presumably due to the stability of the above mentioned oxidative condensation product or the intermediate thereof against oxidative decomposition with insusceptibility to the attack of the oxidizing water-soluble polymerization initiator. The effectiveness of polymer scale prevention is, however, far from satisfactory when a vinyl monomer is polymerized in a polymerization reactor having inner walls coated with the oxidative condensation product of the aromatic amine compound.